The present invention relates generally to milling machines for producing drill screws and methods for producing such products through successive operations of rotary-type saws on a screw blank.
Certain prior art apparatus and methods have been suggested and utilized for producing drill screws and while they have been generally successful and have enabled a commercially accepted drill screw to be provided, the present application contemplates further improvements for facilitating production, minimizing manufacturing costs and providing screws having a more efficient drilling action. The device described in U.S. Pat. No. 3,125,923 typifies the present commercially successful drill screw. The increasing and varying demands for drill screw products requires methods and equipment that are extremely versatile. For example, current drill screw applications include those requiring a long flute length, to be capable of properly drilling into thick materials.
Prior art methods and apparatus for forming drill screws typically utilize saws with a predetermined minimum diameter. For example, a three inch diameter saw is commonly used. This relative size of saws is preferred since the saws themselves are expendable items and are contributory to the cost of production of the drill screws. Furthermore, the saws must periodically be sharpened and it has been found that the particular size saws currently utilized are most efficiently sharpened.
However, the use of such conventional rotary saws creates certain inherent limitations in the methods and apparatus known heretofore. The operations performed to mill the flutes in the blanks include the formation of intersecting arcuate flute surfaces and flat flute surfaces for each flute, with the length of the arcuate surface determining the effective flute length of the drill screw. To maximize the flute length for a given saw diameter, the saw must substantially penetrate the transverse cross section of the blank. This substantial penetration obviously weakens the drill point behind the cutting edges due to the overlapping of saws when forming oppositely disposed flutes. This condition is amplified when a positive axial rake angle is required behind the laterally extending cutting edges. The penetration of each fluting cut an amount greater than half the diameter of the blank at the region of intersection of the positive rake angle surfaces will create a through transverse hole in the shank which is highly undesirable.
Currently successful prior art methods and machinery contemplate either intermittently indexing blanks along a circular path or continuously moving blanks along a circular path. In the intermittent type methods, as typified in U.S. Pat. No. 3,280,412, the blanks are clampingly secured in circumferentially spaced pockets in a turret, preventing the blanks from rotating about their own axis as they are indexed to three different work stations surrounding the circular path. A first work station performs fluting cuts on both sides of the blank simultaneously. Second and third work stations create bevel surfaces on the extremity of the blank to form laterally extending cutting edges intersecting the flute surfaces.
The continuous feed apparatus as typified in U.S. Pat. No. 3,422,472 utilizes a pair of fluting saws at different positions about a circular path created by a turret but contemplates the engagement of the fluting saws with the blank as the blanks themselves are fed past the work station. A pair of pointing saws are also accurately positioned about the circular path to form bevel surfaces on the extremity of the blank, and the blanks themselves are rotated about their own axis between the work stations just described.
Certain other methods have been devised in an effort to produce an improved performance drill screw through the use of a rotary milling cutter apparatus. For example, U.S. Pat. No. 3,780,389 contemplates the orientation of the rotary milling cutter relative to the blank in such a manner so that the generally arcuate flute surfaces extend above and form the laterally extending cutting edge. This design enables a positive rake to be formed behind the cutting edge without fear of overlapping cutters but decidedly limits the effective flute length which can be produced for a commonly used and conventional cutting saw.
The performance and efficiency of drill screws manufactured with the abovementioned milling operations are measured and evaluated by a number of parameters, including the following: the amount of axial pressure required to effect a drilling operation, the time required to drill a hole, the length of flute required to properly exhaust chip material, the configuration and length of the chips produced during drilling, the thickness of the web or column of material remaining between the flute surfaces at a given distance behind a chisel edge, the length of the chisel edge itself, the axial rake angle provided by the flute behind the cutting edge, the amount of relief behind the cutting edge as defined by the location of intersection of the flat beveled surfaces with the flute surfaces opposing the cutting edges. While the prior art methods have produced commercially acceptable drill screws in an efficient and commercially sound manner, in many cases, a compromise of one or more of the above listed parameters has been required because of the lack of versatility of prior art methods and/or machines.